Illico,
thank you so much for making me dig into log files 'n stuff deep enough to discover that...
a) all the HDTV files in hand are correctly advertised as AVC_TS_HD_24_AC3_ISO
b) none of them gets transcoded, therefore there is no ffmpeg command line spawned I'm afraid.
c) the files that don't play properly show numerous "invalid timestamps" errors when analyzed with ffmpeg -i, which I had not at all noticed before.
All files showing "invalid timestamps" errors are those I had remuxed with TSmuxeR somewhere in the process of editing.

Bah!
The "good" files are those which I had just cut and re-saved with DVR-Studio HD (brilliant piece of software btw). They - so far - play flawlessly on the Bravia without transcoding. And if they are not transcoded, they still should be 50 fps, right? If so, there is a
brilliantly short chain of just cutting the original HDTV dvb recording right onto the Serviio machine and streaming it natively to the Bravia.
I played around with various flags/options in TSmuxeR and didn't get a single file that had no "invalid timestamps" errors. Seems I have to look for another muxer to mux additional languages into existing TS files.
Thanks again,
Low